Some of the Facts About Tidal Bores and Where They Occur.Fitness Equipment
Tidal bores are tidal phenomenon that occur when the leading edge of an incoming tide is formed into a wave or waves and this then travels up a river or narrow bay against the normal flow or direction of the current. They occur at a few locations across the world and some can be huge and sometimes dangerous events to witness.
The worlds largest tidal bore occurs in eastern China where the Qiantang River joins Hangzhou Bay. Many locals and tourists flock to this area every September when the famous tidal bore occurs close to the eighth full moon of the Chinese lunar year. 2011 saw the biggest tidal bore for nine years at a height of up to ten metres when it reached its peak on the 15th of that month. An estimated 170,000 people were present to watch the occurrence that began on the 1st of September until the 5th, and then again from the 15th until the 20th. At its height on the 15th many people despite warnings as to the intensity of the strength and height of the wave were swept away due to their being positioned on vantage points in the waves path. This particular bore travels at an estimated 40 kilometres per hour or 25 mph.
Tidal bores only occur during the flood tide and never during the ebb tide. The flood tide appears as a sudden increase in the water level and can be anything from one or two metres and usually up to about six metres. Tidal bores can be produced or take on differing forms and can range from a single wave-front similar to a roller to undular bores that consist of a frontal wave followed by a series of secondary waves. The largest of the world’s bores can be unsafe to shipping or river traffic but welcomed by river surfers.
As well as the turbulence created by the tidal bore it is accompanied by a rumbling noise created by the air bubbles trapped within the eddies created and this roaring sound can be heard at great distances from the river of bay as the acoustics can travel great distances. Another damaging effect or danger from the tidal bores is to the river banks themselves through rapid erosion caused by the sudden increase in water levels.
The word for bore derives from the Old Norse word ‘bara’ through Old English and means wave or swell.
As well as the worlds largest bore as mentioned above Asia also has tidal bores on the Ganges and Indus rivers of India and Pakistan. The Batang Lupar in Malaysia and the Kampar River in Indonesia, the latter event has been known to sink ships up to 130 kilometres or over 80 miles inland. Australia has just two rivers where this occurs in Queensland and the Northern Territory.
Europe has many rivers where this occurrence happens annually with the Severn bore being the most famous when the wave on the River Severn in England and Wales reaches up to 2 metres or over 6 feet in height. In eastern England the Trent Aegir travels up the Humber Estuary at a height of almost 5 feet or 1.5 metres. Many other smaller tidal bores occur across the rivers of the British Isles. In France the phenomenon is called ‘un mascaret’ with the biggest being on the Seine until the 1960’s, due to river dredging and river management it has now been virtually eliminated. Other rivers within the country have smaller tidal bores.
Canada’s Atlantic coast was home to the biggest tidal bore in North America on the Petitcodiac River at over 2 metres. Construction of a causeway and silting of the river reduced the size of the event to little more than a ripple although a restoration project has seen the tidal bore grow in size once again. River management has seen a decrease in size of the tidal bore of the Colorado River into the Sea of Cortez in Mexico while the largest tidal bore in South America is on the Amazon River in Brazil and the Orinoco in Venezuela with recorded heights of up to 4 metres or 13 feet higher than normal, this is called locally ‘pororoca.’ In 2003 a Brazilian surfer rode the longest known distance on a tidal wave staying upright for 37 minutes and travelled 12.5 kilometres inland. The tidal bore on the Qiantang river is so dangerous that nobody had tried to surf it until recently, the longest any one had remained upright on that river was just 11 seconds.